One: Some of the Chinese military hackers who were implicated in a broad set of attacks against the U.S. government and corporations were identified because they accessed Facebook from the same network infrastructure they used to carry out their attacks.

Two: Hector Monsegur, one of the leaders of the LulzSac hacker movement, was identified and arrested last year by the FBI. Although he practiced good computer security and used an anonymous relay service to protect his identity, he slipped up.

And three: Paula Broadwell,who had an affair with CIA director David Petraeus, similarly took extensive precautions to hide her identity. She never logged in to her anonymous e-mail service from her home network. Instead, she used hotel and other public networks when she e-mailed him. The FBI correlated hotel registration data from several different hotels — and hers was the common name.